Semiconductor lasers for the mid-infrared are known since the 1970s. The original devices are edge emitters, pumped electrically in the form of a laser diode, have to be cooled cryogenically, and, depending on design, emit in a spectral range from 2 micrometers to 20 micrometers wavelength. This range will in the following be called the “mid-IR” range. As semiconductor material, narrow gap IV-VI semiconductors like PbSnTe, PbEuTe, PbSrTe, where Te may be fully or partially replaced by Se or S in various compositions, were used.
Later, III-V based lasers using semiconductors like GaAs, GaSb, InP, or InSb and compounds thereof were developed for the mid-infrared, too. Again, these are typically laser diodes that are pumped electrically and edge-emitting. Wavelengths up to about 3.5 micrometers are reached. As an alternative, quantum and interband cascade laser diodes (QCL, ICL), again edge emitters, have proven to be useful. Their spectral range covers wavelengths from about 3 micrometers up to larger than 20 micrometers. QCL and ICL need a sophisticated layered structure, consisting of many hundreds of semiconductor layers grown monolithically on III-V substrates. Some of these designs operate up to room temperature and in continuous wave.
A disadvantage of edge emitting laser diodes is their strong astigmatic emission beam with a very high divergence angle, e.g. 60°, in one direction. To focus or collimate this beam to one with Gaussian-like shape, sophisticated and expensive optical elements are needed, and a considerable part of the output power is lost. The large aperture in one direction is due to the very thin layers of the gain structure in order to keep the threshold current low.
Much is gained if the semiconductor laser emits vertically with respect to the surface. With a typical beam exit diameter of at least several tens of micrometers, a narrow circular output cone results with an opening angle of a few degree only. Such lasers exhibit a “good beam quality” emission in a quasi-perfect radial-symmetric Gaussian shape with very small divergence angles, e.g. below 2°, and with a beam quality factor M2 approaching unity. Contrarily, the factor M2 is much higher in edge-emitting lasers. Such Vertical Cavity Surface Emitting Lasers (VCSEL), also called semiconductor disk lasers, are fabricated monolithically and are often electrically pumped. For higher output powers often an external cavity is used, in so called Vertical External Cavity Surface Emitting Lasers (VECSEL). They are often optically pumped to fully utilize the different construction mechanism to obtain high beam quality, low losses, and high power. Here, the alignment of the pump beam in regard to the spot of the cavity mode is crucial for efficiency. The diameter of both is in the range of few to several hundred micrometers, depending on cavity length, mirror curvatures, emission mode order, and structuring of the semiconductor surface. The alignment and diameter of the pumping spot in regard to the cavity mode spot is essential for efficiency. The misalignment tolerance of the both spots is in the range of a few micrometers, depending on cavity length, mirror curvature, and transversal emission mode order. VCSEL as well as VECSEL are commonly used e.g. for telecommunication applications. While VCSEL are limited to low power applications, very high powers have been reached with VECSEL.
For VECSEL based on III-V semiconductors, the longest laser mid-IR wavelengths are around 3 micrometers. VECSEL using IV-VI materials may, however, cover the whole mid-infrared range. Such devices were described in M. Fill, F. Felder, M. Rahim, A. Khiar, R. Rodriguez, H. Zogg, A. Ishida, IV-VI mid-infrared VECSEL on Si-substrate, Proc. SPIE, 82420H, 2012.
The operating temperature of all lasers in general is commonly stabilized thermo-electrically, in order to reduce noise of output power and emission wavelength. Furthermore, packaging is extremely important for long-term stability and ruggedness. Often, a temperature of light generating semiconductor layers below room temperature, generally also below the dew point, is desired for higher output power or to achieve a different emission wavelength. In this case, special measures need to be taken to avoid water condensation and to reduce heat leakage, for example by operating the laser in vacuum or in a suited dry gas mixture and by a hermetic sealing of the package. Common embodiments enclose the whole laser chip or setup within a standard package where the emission beam exits through a first window, e.g. U.S. Pat. No. 7,572,068, or through an optic component such as a lens, e.g. U.S. Pat. No. 6,953,291. In the case of optically pumped lasers, the pump beam enters the package either through said first or through an additional window, or a fiber delivering the pump beam is fed into the package without breaking the hermetic sealing, e.g. U.S. Pat. No. 6,788,724.
Current optically pumped laser devices are particularly impaired by limitations of available packaging types. Existing packages are designed with electrically pumped devices in mind, but are less than optimal for optically pumped devices. Packages are unsatisfying in regards to device wall-plug efficiency, stability, size, simplicity of the fabrication process, and price. In addition, the long-term stability of the laser characteristics lags behind the requirements of the application, as the thermal and mechanical stability of current packages is insufficient in the case of optical pumping.